P0303 missfire

[BigGeorgesGTP]

ok so i got my car it was in bad shape put NEW.... plug/wires/coil packs/crank posion sensor/ICM/fuel injector and still i get a cylender 3 missfire.. thats the only code and its really bad at idle i just wanna get it running right there is no motor knock or tapping or anything any idea's please help

[Fivefingerdeathpunch]

compression test it yet?

[BigGeorgesGTP]

no im doing that tomorrow... is there anything i should be looking for???

[Fivefingerdeathpunch]

you want all cylinders to be around the same compression and none no lower than 20% less than the average.

[BigGeorgesGTP]

what is the average?

[Fivefingerdeathpunch]

well like say you have:

120
118
114
124
121
and 80

that 80 would be more than 20% less than the average.

Kinda get it, I'm not the best at explaining

[SDGTP99]

It could be a vacuum leak. jus sayin....

[BigGeorgesGTP]

great... im great with cars its my job but im new to the gtp thing lol

[FordMan77]

Check all your vac lines, especially the one T under the SC snout as it likes to rot and crack.

What brand/type of plugs did you install? Gap? These motors do not like platinum plugs and will often have issue with them even when brand new.

Which cyl. got the replacement injector? If it's #3 then it may be worth your while to verify that it is firing properly. IIRC Bill said somewhere that you could bend the tabs on a 194 bulb out and stick the bulb into the injector harness connection and if it lights then it is getting power to fire the injector.

Could be a wiring issue. Also check all your grounds. And since you replace the ICM be sure that the harness to it is all the way in and tight.

[BillBoost37]

I've seen an ICM cause a single cylinder misfire before.

Is the plug fouling or any indication that maybe you have a chipped piston? (compression test may show this) I doubt it's a vaccum leak. Three being the middle cylinder it would have to be a bad injector o-ring and you mention a new injector. However being the middle cylinder, it gets warmer than the outter two and has a higher probability of chipping a piston.

I would check compression and maybe borrow a borescope if possible to look in there. Other than that, swap on a spare ICM (or a buddies) and see if the issue goes away.

[SDGTP99]

^^^This.